Bearing compositions

ABSTRACT

In accordance with this invention there is obtained a polyimidepolytetrafluoroethylene resin composition which can be fabricated into bearings having unusually low frictional resistance.

llnited States Patent ack et 1451 an. 2, 1972 [s41 BEARING COMPOSITIONS 2,400,091 s/1946 Alfthan ..252/12 2,975,128 3/1961 Stott ..252/12 [72] 533 53153 Hams 3,013,967 l2/1961 Rulon-Milleret a] ..252/12 3,126,339 3/1964 Stott ....252/12 [73] Assignee: Dixon Corporation, Bristol, R.l. 3,344,064 9 Brady 81 a 252/122 3,356,759 12/1967 Gerow ..252/12 [221 sew-12,1968 3,453,203 7/1969 Gallagher et al.... 252/12 [211 Appl. No.: 759,527

Primary Examiner-Daniel E. Wyman Related US. Application Data Assistant Examiner-l. Vaughn [63] Continuation-impart of Ser. No. 669,674, Sept. 22,

1 967, abandoned. [57] ABSTRACT [52] [1.8. Ci ..252/12 In accordance with this invention there is obtained a polyi- [51] Int. Cl. ..C 10m 5/20 mide-polytetrafluoroethylene resin'composition which can be [58] Field of Search 1.252/12,12.2, 12.4, 12.6 fabricated into bearings having unusually low frictional resistance. [56] References Cited 23 Claims, No Drawings BACKGROUND OF THE INVENTION Fluorocarbon polymers are well-known materials. Materials such as polytetrafluoroethylene resins, hereinafter referred to as TlFE resins, have achieved wide-spread use for high temperature, dielectric and chemical resistance applications. There has been some utilization of TF E resins for bearing purposes, but such applications have generally been out of necessity rather than desire. Pure polytetrafluoroethylene, when used as a bearing surface or as a rod moving against a bearing surface, appear to wear too rapidly to make their utilization economically justifiable.

Consequently, in the past TFE resins have been provided with filler materials to enhance the wear properties and facilitate the use of TFE as bearing material. Illustrative of filler materials are silicates, such as mica, talc, aluminum silicates and glass. Moreover, the use of silicates in combination with materials such as molybdenum, silver and other metals to reinforce TF E has also been tried with resultant improvement. U.S. Pat. Re. No. 26,088 (Sept. 20, 1966) discloses a variety of filler materials to improve the wear characteristics of Teflon.

Polyimide resins in general have been known for some time although more recently a number of different forms of polyimide resins have been developed. For the most part, polyimide resins can be molded but cannot be used with any degree of success as bearing surfaces. When polyimide resins are used as bearing materials, they offer substantially no protection against excessive scoring and deformation. The polyimides are, however, useful in electrical applications and for other purposes.

THE INVENTION In accordance with this invention, a composition containing TFE resin and polyimide resin can be employed to produce a molded shape having exceptional resistance to frictional wear and hence which is satisfactory as a dry bearing material even against soft metals such as brass. This unusual synergistic interaction between the TFE resin and the polyimide resin is all the more surprising because neither component alone, whether TFE resin or polyimide resin, has satisfactory bearing resistance particularly when moving against a soft metal such as brass.

Thus, this invention also comprises a method of increasing the abrasion resistance of TFE resins.

The polytetrafluoroethylene applicable for use in this invention can initially be in any form, e.g., resin powder, wax, fiber, etc. Telomers of polytetrafluoroethylene are also included within the scope of applicable polytetrafluoroethylene. Polytetrafluoroethylene is obtained by conventional means, for example, by polymerization of tetrafluoroethylene under pressure using free-radical catalysts such as peroxides or persulfates. Representative methods of preparing polytetrafluoroethylene resins are disclosed in U.S. Pat. No. 2,393,967. Polyimide resins are generally prepared by reaction of a primary diamine with an aromatic tetracarboxylic acid, anhydride or ester, to form a linear polyamide-acid which, upon subsequent heating, is converted to the polyimide. The sequence of reactions can be understood more fully by reference to the reaction between pyromellitic acid and tetramethylene diamine. The stages of reaction are as follows:

Stage IFormation of polyamide-acid I H O O C C O OH t( 2)r- 2 IIO O C W C O O H Stage II-Conversion of polyamide-acid to polyimide II In general, the polyimides useful in this invention will have the structural formula in which R is an aromatic ring-containing radical having at least one benzene ring. Preferred R groups include the following:

Phenyl Naphthyl Dlphenyl where R, can be where R is a saturated aliphatic hydrocarbon radical having from one to about six carbon atoms such as In general, R can be any tetravalent aromatic radical having benzenoid unsaturation. The four carbonyl groups are preferably attached directly to separate carbon atoms of aromatic rings. The diamine component of the polyimides can be any diamine known to the art as being useful in the preparation of polyimides. It is well-known that the ultimate properties to be desired influence the nature of the amine that is employed. In general, any poly primary amine is useful to some extent. The amine can be aromatic or aliphatic. Among the aliphatic amines, the diaminoalkanes having at least four carbon atoms in the chain separating amino groups are preferred with the alkane portion having a total of four to about twelve carbon atoms, as, for example, tetramethylenediamine and hexamethylenediamine. Aromatic diamines can also be employed including the phenylene diamines such as rnetaphenylenediamine, various naphthylenediamines and primary amines having the formula NHa R4 in which R. can be in which each R can be lower alkyl of from one to six carbon 5 percent by weight of polyimide resin and conversely more atoms or aryl having about six carbon atoms.

The method of polymerizing to form the polyamide-acids is well-known and does not form a part of this invention. In general, the desired tetracarboxylic acid or anhydride or ester is reacted with a poly primary diamine in a suitable solvent for the requisite time and desired temperature to form the polyamide-acid. The polyamide-acid will generally be soluble in the solvent employed or, if not, will be soluble in one or more other solvents.

The molecular weight of the polyamide-acid and consequently of the final polyimide can vary sharply depending upon reaction conditions and the ultimate properties desired. The preferred polyamide-acids will show a Brookfield viscosity ranging from about 100 to 5,000 centipoises as a percent by weight solution in 1:1 dimethylacetamide and N-methylpyrrolidone at C.

The polyimide can be formed from the polyamide-acid by applying heat to the polyamide-acid, for example, by heating the solution of polyamide-acid in a solvent or by heating the dry solid polyamide-acid. A period of time of at least about 5 to about minutes at a temperature of at least about 400 F. and preferably between about 600 and 700 F. is generally sufficient for conversion of the polyamide-acid to the polyimide. Longer times are, of course, necessary at lower reaction temperatures and also for increased molecular weight and, if desired, for increased cross-linking.

Other methods are also sometimes employed for the conversion of polyamide-acid to polyimide, as, for example, the chemical treatment with a dehydrating agent either alone or in combination with a tertiary amine catalyst. Further details on methods of preparing different polyimide and polyamide-acid resins can be obtained from the literature including, for example, U.S. Pat. Nos. 3,190,856; 3,105,775; 3,179,635; 2,421,024; 3,037,966; 2,710,853; 3,179,634; 2,731,447; and 2,712,543.

There are several methods which can be used to form useful compositions of TFE resins and polyimide. In one method, a polyimide resin can be caused to be finely divided either directly during manufacture or by pulverization to a particle size of between about 10 and 80 microns, preferably between about 70 and 80 microns, and thereafter mixed with a suitable TFE resin also in finely divided form and having a particle size of between about 5 and 50 microns, preferably 25 and 40.

A suitable blending technique should be employed to insure homogeneity. For example, the particulate TFE resin and particulate polyimide resin can be mixed in dry form at room temperature and tumbled to get a fairly uniform mixture. Thereafter, it is generally desirable to add the mixture to a pulverizing machine such as a hammer mill to grind and further mix the resinous components to insure homogeneity. In practice, this has been found desirable to pass the TFE-polyimide mixture through a hammer mill pulverizer having an 16-inch screen (holes are 14; inch diameter). [deal results are achieved when the mixture is passed through the hammer mill 4 times.

An alternate blending method is to add the polyimide resin to an aqueous suspension of TFE resin with agitation to cause the formation of a homogeneous dispersion. Then the precipitation of solids can be caused by the addition of a nonaqueous solvent such as acetone. The mixture thus obtained can be filtered, washed and dried to provide a homogeneous material suitable for molding. Of course, the polyimide resin can be dispersed and the particulate TF E resin added thereto in the converse of the foregoing procedure.

The mixture of polyimide and TFE resins, however obtained, is a very useful product and has exceptional wear characteristics when molded to form a bearing surface. This utility is greater than the utility of either polyimide or TFE resin alone. However, the mixture must contain less than 50 than 50 percent by weight ofTFE resin to insure that the composition has a TFE continuous phase or matrix. In practice it has been found that a mixture of percent TFE resin and 20 percent polyimide provides the most desirable composition material for use in bearings. Additional filters can also be added during the blending stage to add whatever properties such fillers normally would be expected to impart to resin compositions. Generally, not more than about 15 percent by weight of the mixture should be composed of such other fillers which can include graphite, glass fibers, metallic oxides, mica, talc, bronze, copper, aluminum, silver, molybdenum, Fiberfrax, and the like.

By careful control of proportions, relative particle sizes, degree of homogeneity and fabrication conditions, compositions of this invention can be prepared which will afford optimum bearing characteristics.

In this invention by which TFE resins are rendered more abrasion-resistant, TFE resins are improved through the incorporation of from about 5 to 40 percent by weight, preferably at least 10 percent and ideally 20 percent of polyimide resins while maintaining the polytetrafluoroethylene as the continuous phase or matrix. The TFE and polyimide resins can be blended directly in dry form or the polyimide resin powder can be mixed with an aqueous dispersion of TFE resin particles. The particle sizes and relative bulk volumes should be such as to maintain the TFE as the continuous phase or matrix, e.g., by using relatively smaller sized TFE resin particles.

The compositions of this invention can be used for forming molded objects of varying types. The primary utility of the compositions would appear to be as bearings and as bearing surfaces. However, other useful molded products can also be made of these materials. The method of molding should be apparent to those skilled in the art of molding polymeric materials. A representative technique for preparing bearings is the following:

A steel mold approximately 12 inches long is used to preform a slug or a small billet that is ultimately formed into a bearing. A 1 inch long end plug is contained at the bottom of the mold. The mold is filled with the powdered composition of this invention. A l-inch-long end plug is loosely placed on the top and a pusher rod is installed to facilitate pressurization of the billet. The mold is then pressurized to about 6,000 p.s.i. to perform the billet. Subsequently, the mold is placed into an oven which has been preheated to 700 F. The mold is kept in the oven for 3 hours during which time the oven is maintained at the 700 F. temperature. The mold is then removed from the oven and a pressure 3,000 p.s.i. is applied while the mold and billet are allowed to cool. The molded piece is then cut, drilled and bored to size.

Further details on the processes and products of this invention can be obtained from the following examples, which represent the best mode of the invention known to the inventors:

EXAMPLES 1 to 4 In each of the following examples the starting material used to produce the polyimide was a polyamide-acid commercially obtainable in the form of a polyamide-acid varnish composition. The polyamide acid is prepared by the reaction of benzophenone tetracarboxylic acid anhydride with 4,4'-oxydianiline in a solvent of N-methylprrolidone in accordance with the procedure of U.S. Pat. No. 3,190,856.

The polyimide resin was prepared from the commercial varnish by pouring the material into a vacuum reactor, applying 28 inches 1/31vacuum and then allowing air to flush into the reactor to achieve 18 inches of vacuum and maintaining a level of 18 inches of vacuum. The reactor was heated for ap- PVT where P is pressure in p.s.i. on the projected area, in this proximately 16 hours at 225250 F. followed by passing the case 1-square inch, V is speed in feet per minute, and Tis time material through a Cumberland grinder using a Aa-inch screen. in wear tests are u r at l ast 150 h urs. The material was then heated unde 15 i h f vacuum at The test results recorded in Table I show surprising benefits 225-250 F. for an additional 8 hours and ground in four 5 obtained by means of this invention. Thus, bearing surfaces passes through a Mikro-pulverizer with a "-inch screen. made of P polyimide p P TFE resin Thereafter, the material was dried in air at 425 F. for about 1 ample Could not be Operated for the time necessary for the hour to remove all solvent. After solvent removal was test- The P polyimide beating Sutface, Example 1, failed in complete, the temperature was raised to for 15 less than 5 minutes of operation, resulting in severe scoring of minutes to permit conversion of the po1yamide aeid to the the test drum due to the absence of a satisfactory bearing surpe|yimide form The polyimide product was theyl passed face. Useof the pure TFE bearing surface was discontinued through the Mik l i ith a 32 Screen after minutes because it, too, was at the point of failure and A Series f compositions were made up having varying any further use would have resulted in scoring. No wear factor amounts of TFE resin and polyimide resin. In forming blends could even be calculated for the polyimide resin bearing of THE resin and polyimide resin the selective quantity of 15 whFreas a wear factor f 50,000 could roughly be polyimide resin as prepared as above and the selective quantiesttmated e TEE Y bearmg' e when TFE ty of TFE resin, in each case a commercial fine-grind TFE [esm Provlded varymg amwmsfif polylm'de were powder having a particle size of about 30 microns, were mixed Ployed under prefilsely the Same condmonst much lower wear together at room temperature and tumbled to get a fairly factors were obtamed' uniform mixture. Thereafter, the mixture was pulverized in a Similarly, an tpectedly low coefficient of friction was pulverizing machine to grind and further mix the TFE and *fl P$ 9! FEW PP Y polyimide resins. The particular blends prepared are set forth in Table lbelow. TABLE I Bearings and other test shapes were then formed of the Example 1 2 3 4 powder by pressure molding techniques in which an arr Pol hide resin msb Wei ht 100 20 10 0 operated ram forces a charge of powder down into a die or 1 Resin g f 80 90 100 mold. Various h sical tests were condu t d m l d Specific gravity T H0'I) p y c e on the ode Tesg sli le strength, p.s.i. (ASTM 1708- object in accordance with the procedures set forth in Table I 11,600 2' 470 4,000

and the results are recorded in Table I. gtsg i g g (AS'II\ 'I HOB-12 T) 2 3 0 The coefficient of friction value recorded in Table I was MST? if g determined by molding the particular composition into test at: 24 at R T memcient sleeves measuring )2 inch [.D. X 1 1/16 O.D. X '2: inch long and w fg b' 'ii press fitting the sleeves into aluminum housings having a 2 10,090 at 0) 30 50 3 50.000

inch O.D., thereby forming a test bearing with the bearing sur- 1 Excessive flaking.

face being of a composition specified in Table I. The test bear- 2 Failed Within 5 minutes' 3 Discontinued after 15 minutes. mg was then placed on a -inch-diameter shaft driven by a variable speed motor. This shaft is adjusted to a maximum indicator run out of 0.001 inch. The bearing is loaded by placing EXAMPLES 5 to 13 a weight connected to a wire around the groove machined on 40 the circumference of the aluminum housing. Frictional force Illustrate the P the P P i mvemlon as is then measured by a Brabender dynomometer. in the stanbearmg surfaces use m f f e j metals dard tests, samples are run for one hour at a pressure velocity such as brass varfous TFE composmons wnh polyfmtdc value of 2,000 using a load of approximately 100 pet at a were prepared as m Examples 1 through 4, the polyimide resin shaft surface speed of approximately 20 feet per minute. The bemg the e as used f Where fillets t added they initial clearance on the bearing is set to between 0.003 and were added slmultaneously with the y blendlng of the TFE 0.006. The coefficient of friction is then calculated following e polyimide Proportions of ingredients e Set forth completion f the test o emu-Sey the lower the ffi i t f in Table II. In calculating the wear factor set forth in Table 11, friction h more l bl h b i the shaft material employed in the tests in determining the in determining the wear factor K, set out in Table I, th coefficient of friction and the wear factor was brass and in calsame bearing, housing and shaft are used as in the friction test. l ing h weal factor a PVOf 5,000 was used at a load f A PV of 10,000 is used at a load ofapproximately 45 p.s.i., approximately 23 p.s.i. with the shaft surface speed being 220 and a shaft surface speed of 220 feet per minute is employed. feet per minute. The results obtained are recorded in Table [1.

TABLE II On brass shalt Parts by weight Condition Poiyirnide Coefficient Wear of shaft resin Filler. it any of friction factor after test Flaked too badly for use. K 50,000 36 and. 1 D0. 0.18 2 Do. 0. 26 20 Do. Failed within 5 minutes 1 200 Goodv 5 D0. Failed within 5 minutes 1 1 Contains 15% by weight of a 'IFE wax composition. 9 With excessive scoring of shaft.

The shaft material is made out of drill rod of a hardness of Again, from Table II, it is immediately apparent that a comfrom B to Rockwell. In calculating the wear factor K, the position of TFE with polyimide resin yields results conweight loss after running for a given period of time is detersiderably different in kind than the results obtained from mined when used to compute radial wear by using the formula either TFE or polyimide resins alone as bearings. Thus, TFE R W d X A. Wis the weight loss and A is projected area and d resin bearing surfaces flake too badly for use and pure polyiis density. K is then calculated by means of the formula K R rnide resin failed almost immediately when put into service.

EXAMPLES 14 to 18 Examples 14 to 18 illustrated the wear proprieties of TFE having polyimide incorporated therein as compared with standard glass and graphite filled TFE formulations. As can be seen from Table III, three TFE formulations were tested for wear. The three formulations were respectively, TFE having 20 percent polyimide incorporated therein, TFE with a 25 percent glass filler and TFE with a 15 percent graphite filler. Each of the test specimens were sleeves which were formed of %-inch-diameter rods preformed at 8,000 p.s.i. and sintered using a standard SPI cycle. The three formulations were arranged on shafts made respectively of brass, aluminum, aluminum (Martin Hard Coat), 316 stainless steel and drill rod. ln each case the PV factor was raised from a minimum to a maximum and K factors (wear factors) and shaft conditions were examined for various PV levels. The examples, as seen in Table lll show the results of the examples indicating the superiority of Teflon having polyimide incorporated therein over glass or graphite filled TFE.

ing materials for use against soft metals such as brass. It can be readily understood that in many instances the use of a lubricated bearing surface is not desirable. However, until the present time there was very little alternative to the use of lubricated bearings for surface in bearing contact with soft metals.

Similar satisfactory bearing compositions were obtained using mixtures of tetrafluoroethylene resin and the linear polymeric amide modified polyimides of US. Pat. No. 3,179,635 including the commercial product designated as Westinghouse Al-l 31 believed to be the product produced in accordance with Examples l and 2 of the said patent. Satisfactory results were also obtained with polyimide blends in which the polyimide was Monsanto RX-268 and Monsanto TS-6720 and Amoco Al-ll polyimide resins.

In addition to these specific polyimide resins, any of the polyimide resins known to the art including those described above can be employed since it appears to be not the nature of the total molecule but rather the presence of the imide groups that leads to the unexpected results obtained by means of this invention.

We claim: 1. A method of making a composition useful for making bearings comprising the steps of:

TABLE III Type of filler g ass grap 1 e 2 P013" Type of shaft PV K lmide shat K Shaft K Shaft Example 14 Brass 5 000 Example 15 Aluminum Example 16 Aluminum (Martin hard coat).

Example 17 316 s. s 15,000

20,000 120 S1. scored Example 18 Drill rod 8 888 8% g gi 60 OK 'Failtd" means that the bearing bound and the shaft became badly scored. This usually occurred in less than 1 hour of test time.

EXAMPLES l9 and 20 To determine the effect of the particle size of the polyimide material incorporated in the TFE formulations tests were conducted using l00 mesh (149 micron) size polyimide particles and 200 mesh (74 micron) size particles in a TFE formulation having 20 percent polyimide incorporated therein. The TFE formulations were formed into sleeves made from =36-inch rods which were preformed at 8,000 psi. and sintered using the standard SPl cycle. The tests indicate that the wear factors of 20 percent polyimide TFE formulations are enhanced if the 200 mesh (74 micron) size particles are used. Table IV illustrates the result of the test.

The results of this invention yield excellent bearing compositions for soft metals such as brass. Until this discovery, the art had no satisfactory high temperature non-lubricated bearin which R is a tetravalent aromatic radical; R is an organic radical derived from an organic poly primary amine and n is a number taken to produce the desired molecular weight;

agitating the mixture to form a homogeneous dispersion;

and

precipitating the solids from said aqueous suspension by adding a nonaqueous solution.

2. A method as in claim 1 wherein the proportion of polyimide resin is 20 percent by weight of the composite polyimide and polytetrafluoroethylene mass.

3. A method as in claim 2 wherein the polytetrafluoroethylene resin suspended in the aqueous suspension has a particle size of between 5 and 50 microns; and

the polyimide resin added to the aqueous suspension has a particle size of between 10 and microns.

in which R is a tetravalent aromatic radical; R is an organic radical derived from an organic poly primary amine and n is a number taken to produce the desired molecular weight.

7. A composition as in claim 6 wherein R is an aliphatic group containing from four to 12 carbon atoms.

8. A composition as in claim 6 wherein R is an aromatic group.

9. A composition as in claim 8 wherein R is a phenylene group.

10. A composition as in claim 8 wherein R has the formula. I

in which R is selected from the group consisting of in which each R V .-.M group is a lower alkyl group or an aryl group having up to about six carbon atoms.

11. A composition as in claim 8 wherein R is a naphthylene group.

12. A composition as in claim 6 in which R is an aromatic radical containing at least one benzene ring.

13. A composition as in claim 6 in which R is selected from the group consisting of phenyl, naphthyl, diphenyl and the group where R is a member from the group consisting of 6, S O1, a,

O R and O R O and R is a saturated aliphatic hydrocarbon having from one to about six carbon atoms.

14. A composition as in claim 8 in which R is selected from the group consisting of phenyl, naphthyl, diphenyl and the group where R is a member from the group consisting of SO:. Ra, Q M "he. O R and O R O and R is a saturated aliphatic hydrocarbon having from one to about six carbon atoms.

15. A composition as in claim 9 in which R is selected from the group consisting of phenyl, naphthyl, diphenyl and the group where R is a member from the group consisting of -C-, "5-, -O, S Oz, R3,

- O R and O R O and R is a saturated aliphatic hydrocarbon having from one to about six carbon atoms.

16. A composition as in claim 6 in which the polyimide is present in an amount of at least 5 percent based upon the combined weight of polyimide and tetrafluoroethylene polymer.

17. A composition as in claim 15 in which the polyimide is present in an amount of at least about 10 percent by weight based upon the combined weight of polyimide and tetrafluoroethylene polymer.

18. A composition as in claim 15 containing 20 percent by weight of polyimide and percent by weight of tetrafluoroethylene polymer.

19. The composition of claim 6 in the form of a bearing.

20. A composition as in claim 8 containing a continuous phase of polytetrafluoroethylene and a discontinuous phase comprising the polyimide produced by reacting pyromellitic acid or the anhydride thereof with a diamine.

21. A composition as in claim 20 wherein the diamine is tetramethylene diamine.

22. A composition as in claim 20 containing a continuous phase of polytetrafluoroethylene and a discontinuous phase comprising the polyimide produced by reacting benzophenone tetracarboxylic acid or the anhydride thereof with a diamine.

23. A composition as in claim 22 wherein the diamine is oxydianiline. 

2. A method as in claim 1 wherein the proportion of polyimide resin is 20 percent by weight of the composite polyimide and polytetrafluoroethylene mass.
 3. A method as in claim 2 wherein the polytetrafluoroethylene resin suspended in the aqueous suspension has a particle size of between 5 and 50 microns; and the polyimide resin added to the aqueous suspension has a particle size of between 10 and 80 microns.
 4. A method as in claim 3 wherein the nonaqueous solution added to precipitate the solid from said aqueous solution is acetone.
 5. A method as in claim 3 including the further steps of: filtering the precipitate of said aqueous suspension; washing the precipitate; and drying the precipitate.
 6. A composition useful for making bearings comprising a continuous phase of a tetrafluoroethylene polymer having dispersed therein 5-40 percent by weight of a polyimide resin of the structural formula in which R is a tetravalent aromatic radical; R'' is an organic radical derived from an organic poly primary amine and n is a number taken to produce the desired molecular weight.
 7. A composition as in claim 6 wherein R'' is an aliphatic group containing from four to 12 carbon atoms.
 8. A composition as in claim 6 wherein R'' is an aromatic group.
 9. A composition as in claim 8 wherein R'' is a phenylene group.
 10. A composition as in claim 8 wherein R'' has the
 11. A composition as in claim 8 wherein R'' is a naphthylene group.
 12. A composition as in claim 6 in which R is an aromatic radical containing at least one benzene ring.
 13. A composition as in claim 6 in which R is selected from the group consisting of phenyl, naphthyl, diphenyl - O - R3 -, and - O - R3 - O -, and R3 is a saturated aliphatic hydrocarbon having from one to about six carbon atoms.
 14. A composition as in claim 8 in which R is selected from the group consisting of phenyl, naphthyl, diphenyl - O - R3 -, and - O - R3 - O -, and R3 is a saturated aliphatic hydrocarbon having from one to about six carbon atoms.
 15. A composition as in claim 9 in which R is selected from the group consisting of phenyl, naphthyl, diphenyl - O - R3 -, and - O - R3 - O -, and R3 is a saturated aliphatic hydrocarbon having from one to about six carbon atoms.
 16. A composition as in claim 6 in which the polyimide is present in an amount of at least 5 percent based upon the combined weight of polyimide and tetrafluoroethylene polymer.
 17. A composition as in claim 15 in which the polyimide is present in an amount of at least about 10 percent by weight based upon The combined weight of polyimide and tetrafluoroethylene polymer.
 18. A composition as in claim 15 containing 20 percent by weight of polyimide and 80 percent by weight of tetrafluoroethylene polymer.
 19. The composition of claim 6 in the form of a bearing.
 20. A composition as in claim 8 containing a continuous phase of polytetrafluoroethylene and a discontinuous phase comprising the polyimide produced by reacting pyromellitic acid or the anhydride thereof with a diamine.
 21. A composition as in claim 20 wherein the diamine is tetramethylene diamine.
 22. A composition as in claim 20 containing a continuous phase of polytetrafluoroethylene and a discontinuous phase comprising the polyimide produced by reacting benzophenone tetracarboxylic acid or the anhydride thereof with a diamine.
 23. A composition as in claim 22 wherein the diamine is oxydianiline. 